Polymeric coatings having specific release properties toward adhesives are widely used. Silicones such as polydimethylsiloxanes, polymers containing predominantly dimethylsiloxane units, are frequently employed as release coatings for products such as labels or large adhesive-coated sheets sold in other than roll form. Notwithstanding a relatively high cost, such polymers are used for these applications because of their capability of providing coatings of very low release value, e.g., in the range 0.4-1.6N/100 mm width. The term "release value" refers to the amount of force required to peel a strip of pressure-sensitive adhesive tape from a surface.
Polydimethylsiloxanes are less useful, however, as release coatings on the back surface of adhesive tape, because their low release force can cause roll instability and handling problems. Such a coating on the back surface of a tape is often referred to as a low adhesion backsize or LAB. LABs for tapes in roll form ideally exhibit release (or release value) toward the adhesive of about 6 to 35N/100 mm width. Polymers with higher release values make it increasingly difficult to use the tape, and delamination of the adhesive from the substrate often can result. Many non-silicone polymers, e.g., certain types of polyurethanes, find use as low adhesion backsizes for pressure sensitive tapes because of their much higher release value than that of the polydimethylsiloxanes, typically greater than 20N/100mm width. Such non-silicone LAB coatings are exemplified in U.S. Pat. Nos. 2,532,011, 2,607,711, 2,876,894 and 3,342,625.
For products such as tapes and liners, coatings having specific release properties toward adhesives which are intermediate between those of the polydimethylsiloxanes and conventionally used non-silicone LAB coatings are highly desired. Many previous attempts to provide such coatings by modification of polydimethylsiloxanes or by blending them with less effective release material, as disclosed in, for example, U.S. Pat. Nos. 3,328,482, 3,527,659 and 3,770,687, have not met with total success. Some of the resultant coating compositions so contaminate a pressure-sensitive sensitive adhesive that it loses its tack, while others gradually react with pressure-sensitive adhesives in such a manner that they cannot be separated after remaining in contact for an extended period of time. Other compositions are difficult to reproduce with consistency, exhibit changed release properties as the silicone gradually migrates to the surface, or require cure temperatures so high that curing adversely affects the substrate on which it is coated.
Some of these problems can be eliminated or reduced by the use of radiation curing (i.e., curing via ultraviolet light or electron beam) rather than thermal curing of silicone release coatings. Radiation curing of polydimethylsiloxane-containing coatings has been recently discussed by D. J. Cyterski in the Technical Seminar Proceedings of the Pressure Sensitive Tape Council (May 8-10, 1985), pages 45-53. Reference is made to the advantages of radiation curing in making possible the siliconization of heat-sensitive substrates and in producing coatings which often possess more stable release characteristics than those of thermally-cured coatings. According to this discussion, however, attempts at modification of these radiation-cured compositions to provide controlled release over a range of values have, to date, been only partially successful. In addition, the curing requirement itself is a disadvantage since it adds cost and additional steps to the coating process.
U.S. Pat. No. 4,171,397 discloses a composite release surface presenting areas of (1) fluorochemical polymer and (2) cured silicone polymer, and controllably providing release values in the range 0.8-35 N/100 mm width, requiring double coating and curing.
Coating compositions exhibiting intermediate release properties toward adhesives have been provided by reaction of an isocyanate with a hydroxyl- or amine-containing organosiloxane (U.S. Pat. No, 3,997,702) and by the use of epoxypolysiloxanes and their blends with epoxyterminated silanes (U.S. Pat. No. 4,313,988). The former method provides a coating which is effective with only a few specific adhesives, and, since these are crosslinked materials, both methods require a curing step.
Graft copolymers, some containing silicone, are being increasingly used for general modification of surface properties, as is described in a series of papers by Y. Yamashita et al., [Polymer Bulletin 7, 289 (1982); Polymer Bulletin 10, 368 (1983); Makromol. Chem. 185, 9 (1984); Macromolecules 18, 580 (1985)]. Such use is also reflected in some recent Japanese art, such as Japanese Patent Application No. 57-179246, published November 4, 1982, which concerns the use of graft copolymers as additives to impart hydrophobicity (or hydrophilicity) to surfaces. In Japanese Patent Application Nos. 58-167606, published Oct. 3, 1983, and 58-154766, published September 14, 1983, a method of preparation of silicone graft copolymers and the use of these copolymers in coating compositions such as paint films are described. Here, the copolymers provide water- and oil-repellency, stain resistance, and reduced frictional properties. Japanese Patent Application No. 59-78236, published May 7, 1984, discloses a method of preparing monofunctional polymeric silicone monomers, i.e., macromonomers, for use in the preparation of graft copolymers as surface treatment agents. The use of such silicone macromonomer-grafted copolymers in coating compositions, again to impart water- and oil-repellency, stain resistance and low friction characteristics, is described in Japanese Patent Application No. 59-126478, published July 21, 1984.
None of the aforementioned, however, teaches the use of chemically tailored, polysiloxane-grafted copolymers in controlled release applications involving pressure sensitive adhesives.